1. Field of the Invention
The present invention pertains to electric cable terminations, and in particular, to terminations for electric cables used for electric power distribution.
2. Description of the Related Art
Electric cables having concentric neutral conductors and embedded wire conductors are becoming increasingly popular. For example, such cables are used in direct burial, Underground Residential Distribution (URD) applications. These cables are typically operated at thousands of volts, and hence, care must be taken in managing or controlling the electrical stress at points where the cable is terminated for coupling to electrical components such as switchgear units, for example.
Such cables typically include electrical shielding surrounding the central current-carrying conductor. For example, cables can include a concentric semi-conducting jacket, concentric metallic foil wrapping, or concentrically wrapped, discrete drain wires. The various arrangements of semi-conducting and conducting layers surrounding the central current-carrying conductor control the electric stress and induced electrical fields surrounding the central conductor, features which are important for direct burial and the like applications.
Cable terminations typically require removal of the various dielectric, semi-conducting and conductive shield layers surrounding the central current-carrying conductor with the various semi-conducting and conducting surrounding layers being appropriately connected to the electrical equipment involved. For example, a porcelain termination kit is available from the Minnesota Mining and Manufacturing Company as SCOTCHCAST 5903. The kit, which is field-installed, provides termination for concentric neutral URD cable having a semi-conducting jacket surrounding the cable insulation, disposed within a helical wrap of much smaller gauge concentric wires, commonly referred to as drain wires. The various layers of the concentric neutral cable are removed in the field following closely dimensions laid out in the installation instructions accompanying each kit.
For example, the drain wires are pulled back to expose a predetermined length of the cable semi-conductive jacket. A bottom end cap is inserted over the semi-conductive jacket and forms a lower seal therewith. The semi-conductive jacket is cut back at a predetermined distance from the cable tip, as is the cable insulation, exposing a predetermined length of the bared current-carrying conductor. A series of different tapes are wound about defined sections of the prepared cable end. Vinyl plastic electrical tape, semi-conducting tape, and stress control tape, for example, are employed.
A porcelain insulator is disposed about the prepared cable end and a mounting ring is slid underneath the porcelain insulator before its insertion in the bottom end cap. A top cap includes a recess for receiving the current-carrying conductor, and includes a threaded hole for receiving a pour spout. An elastomeric compound commercially available as SCOTCHCAST 2100 is poured through the top cap to fill the interior of the porcelain insulator. The hole is plugged with a threaded sealing plug. The entire assembly of the termination, including pouring of the elastomeric compound is performed in the field and is therefore subject to different temperature and humidity conditions, for example.
It has been found during development of the present invention, that the first 20 hours or so of the curing of an elastomeric compound is particularly critical to the subsequent performance of that cast compound in a high voltage electrical insulation system. As those skilled in the art will recognize, it is difficult to maintain carefully controlled conditions in the field, let alone for an extended period of time such as a 20-hour period subsequent to the casting of the elastomeric compound. As described, the cable is inserted in the porcelain insulator before the elastomeric compound is poured and thus, the resulting cured compound structure takes on the diameter of the cable portions disposed within the porcelain insulator. Experience has shown that elastomeric fillers applied in the field do not offer good performance at low temperatures and that due to different coefficients of thermal expansion, a gap between the filler and the cable might result.
A terminator commercially available from Joslyn Manufacturing Company, Stock Item #E5200, provides a porcelain termination with a stress relief cone which is loaded inside the porcelain insulator at the lower end thereof, and an elastomer sleeve is inserted between the prepared cable end and the porcelain insulator bore. In order to eliminate air gaps between the sleeve and porcelain and the sleeve and cable insulation, a compression spring of considerable strength is loaded into the top of the porcelain insulator, surrounding the upper end of the prepared cable end. An upper cap is secured to the porcelain insulator to maintain the spring in compression. A relatively large number of components and a higher level of expertise are required for assembly. Further, neither the stress relief cone nor the elastomer sleeve bond to the porcelain insulator and thus, air voids at the interface between these members and the porcelain insulator may ultimately lead to dielectric failure.
A porcelain termination sold by the G & W Electric Company, assignee of the present invention, is commercially available under the trade designation "Slip-On Terminator." This termination employs a porcelain insulator which slips over a prepared cable end. A length of sponge tubing is inserted between the prepared cable end and the inner bore of the porcelain insulator, and also in a mounting base which is disposed below the insulator, also surrounding the prepared cable end. The sponge does not bond to the inner bore of the porcelain insulator and greater retention strength at low temperatures is desired.
As mentioned above, it is important to control the electrical stress at the prepared cable end. Of particular interest is the control of electrical stress at abrupt changes in the cable shielding system. One area of concern is the point where the semi-conducting layer is cut away, thus creating a discontinuity in the electrical field surrounding the cable. Heretofore, measures have been taken to directly bond metallic base mountings at the bottom end of a cable termination, so as to position the ground plane at a portion of the cable termination which is designed to handle the electrical stress. It is desirable to provide a ground plane at the bottom of the cable termination which extends through the various termination components and which adapts to thermal changes in cable size, to eliminate interstices that may form between the various termination components, particularly the cable and a metallic support base.
Further improvements in metallic support bases are desirable to ensure hermetic sealing between the metallic base and the porcelain insulator resting thereon. To be commercially attractive, such hermetic sealing should be easy to install and economical to fabricate. Heretofore, oil-filled terminations have been provided to address many of the above concerns. For a variety of reasons, which have long been recognized in the industry, it is desirable to eliminate liquid-filled terminations.